We wish to elucidate the mechanisms which control phospholipid metabolism by studying systems in which that metabolism can be experimentally manipulated. Such systems are the thyroid gland which can be stimulated by thyrotropin, Ehrlich ascites tumor cells in culture which respond to serum additions to their growth medium, and several organs responsive to hormonal control (e.g. pancreas and acetycholine, renal cortex and parathormone). Concomitant objectives are to further elucideate the mechanism of action of thyrotropin (TSH) and the control of growth in malignant cells. In all the above systems, and particularly in the thyroid, phosphatide turnover as measured by PO32 sub 4 (and in some cases labelled glycerol) incorporation can be stimulated by the agents mentioned above. This allows examination of the stimulated and unstimulated state and by comparison of those states allows a study of control points. The general method employed is the study of the details of the individual steps of the biosynthetic pathways of the phosphatides and other pertinent substrates and lipids. This is accomplished by use of tissue incubations with appropriate labelled compounds, lipid extractions, lipid fractionations by thin layer chromatography, radioactive assays, and chemical analyses. Comparison of results between the effects of the stimulating agent on different steps and substrates and between different tissues should lead to an understanding of general control mechanisms.